Progressive cavity pump (PCP) systems are used for artificial oil lifting operations on wellheads. The PCP systems have a drive head at the surface and a rotor and stator downhole. The drive head rotates a rod string that turns the rotor in the stator. This lifts a fluid column up a tubing string to be produced at the surface.
The PCP systems tend to store energy in the rod string and the lifted column of fluid. This stored energy can be problematic if the release of the energy is not controlled properly when the well is shut off. Various breaking and decelerating devices have been developed for surface drive heads to control the release of the stored energy. Unfortunately, current devices can be expensive and may not be effective in every situation.
One downhole device for dealing with the stored energy uses a dump valve to direct fluid out of the tubing to the annulus. When opened, the dump valve prevents the column of fluid from going through the pump and generating hydraulic energy that causes backspin on the rod string. Another downhole device uses a check valve at the pump intake. The check valve holds the weight of the fluid column above the pump and keeps it from going through the pump and generating the hydraulic energy that causes backspin on the rod string. Although these downhole devices may deal with the problem, these devices can create improper rotor spacing and can reduce the pump's efficiency. Moreover, if these downhole devices fail, then operators must deal with the full stored energy.
The most common devices to control the release of the stored energy are used at the surface. Various surface devices can use braking to control the release of stored energy in the rod string. The braking can use direct mechanical braking, hydraulic braking, centrifugal braking, or the like at the surface drive head. However, one major limitation to the surface devices is their inability to dissipate the tremendous amount of heat that they can produce. For example, the ISO standard for PCP drive heads may require a temperature below a certain limit (e.g., 150° C.) during backspin. The defined limit can eliminate the feasibility of using certain braking devices due to the large amount of energy that could potentially be stored in the fluid column filling the tubing.
To overcome the thermal limitations of such surface devices, operators have designed oversized equipment, which increases costs. Operators have also designed the surface devices to limit the reverse backspin velocity that can be achieved when controlling the release of the stored energy. For example, systems may use a variable speed driver (VSD) on the permanent magnet or induction motor to apply torque during backspin. To use these systems during a power blockout, the system needs either permanent magnets or additional capacitors. In another example, the surface device may use a small choke in a hydraulic brake. However, this solution has a negative impact on the operation of the PCP system because it increases the amount of time required to release the energy before production can be resumed or before well intervention can be initiated.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.